Vertical cavity surface emitting lasers (VCSELs) based on inorganic semiconductors (e.g., AlGaAs) have been developed since the mid-80's (K. Kinoshita et al., IEEE J. Quant. Electron. QE-23, 882, 1987). They have reached the point where AlGaAs-based VCSELs emitting at 850 nm are manufactured by a number of companies and have lifetimes beyond 100 years (K. D. Choquette et al., Proc. IEEE 85, 1730, 1997). With the success of these near-infrared lasers in recent years, attention has turned to other inorganic material systems to produce VCSELs emitting in the visible wavelength range (C. Wilmsen et al., Vertical-Cavity Surface-Emitting Lasers, Cambridge University Press, Cambridge, 2001). There are many fruitful applications for visible lasers, such as display, optical storage reading/writing, laser printing, and short-haul telecommunications employing plastic optical fibers (T. Ishigure et al., Electron. Lett. 31, 467, 1995). In spite of the worldwide efforts of many industrial and academic laboratories, much work remains to create viable laser diodes (either edge emitters or VCSELs) which span the visible spectrum.
In the effort to produce visible wavelength VCSELs, it would be advantageous to abandon inorganic-based systems and focus on organic-based laser systems. Organic materials have properties making them suitable for gain media in these lasers, such as low scattering/absorption losses and high quantum efficiencies. Organic lasers offer the advantage over inorganic systems in that they are relatively inexpensive to manufacture and can be made to emit over the entire visible range.
The usual route for making a manufacturable laser diode system is to use electrical injection rather than optical pumping to create the necessary population inversion in the active region of the device. This is the case for inorganic systems, since their optically pumped thresholds for broad-area devices are on the order of 104 W/cm2 (P. L. Gourley et al., Appl. Phys. Lett. 54, 1209, 1989). Such high power densities can only be obtained by using other lasers as the pump sources, precluding that route for inorganic laser cavities. Unpumped organic laser systems have greatly reduced combined scattering/absorption loss (˜0.5 cm−1) at the lasing wavelength, especially if a host-dopant combination is used as the active media. As a result, optically pumped power thresholds below 1 W/cm2 should be attainable, especially when a VCSEL-based microcavity design is employed in order to minimize the active volume (which results in lower thresholds). At these threshold power levels it becomes possible to optically pump organic-based vertical laser cavities using incoherent light-emitting diodes (LEDs). This result is highly significant for amorphous organic laser systems, since driving them by electrical injection has, to this date, been unobtainable mainly as a result of the low carrier mobility of organic materials (N. Tessler et al., Appl. Phys. Lett. 74, 2764, 1999).